Quick answer

Vitamin C has low toxicity, and NIH says high intakes are not believed to cause serious harm in healthy people. The usual complaints at high doses are loose stools, nausea and stomach cramps. Adults should stay under 2,000 mg a day from all sources, and a few groups should ask a doctor first.^1,2

Key facts

The upper limit is 2,000 mg a day for adults and 400 mg for children aged 1 to 3.¹
Stomach upset at high doses comes from vitamin C that is not absorbed, which draws water into the gut.¹
At 1,000 mg a day, the oxalate and urate passed in urine rose in an NIH study of healthy volunteers.³
Some effects once blamed on high intakes, such as lower vitamin B12, were never confirmed.¹

The most common side effects

High doses can cause diarrhea, nausea, stomach cramps and other gut upset, because vitamin C that is not absorbed draws water into the gut.^1,2

The upper limit for adults is 2,000 mg a day from food and supplements combined, including during pregnancy and breastfeeding. It is 1,800 mg for teens aged 14 to 18 and lower for children. It does not apply to people given vitamin C as medical treatment by a doctor.^1,2

Upper limits for vitamin C by age, from all sources^1,2
Age	Upper limit per day
1 to 3 years	400 mg
4 to 8 years	650 mg
9 to 13 years	1,200 mg
14 to 18 years	1,800 mg
Adults	2,000 mg

No upper limit is set for babies under a year old. NIH says their vitamin C should come only from breast milk, formula and food.¹

Kidney stones

High intakes raise the oxalate and uric acid passed in urine, which could add to kidney stones, especially in people with kidney problems. Studies conflict, and NIH says the best evidence of a link is in people who already pass too much oxalate.^1,3

If you have had kidney stones, or have kidney disease, ask your doctor before taking vitamin C above the recommended amount.

Iron overload

In people with hereditary hemochromatosis, high doses taken for a long time could worsen iron overload and damage tissues. NIH says people with this condition should avoid vitamin C supplements.^1,4

Hemochromatosis is inherited. If it runs in your family, ask your doctor before taking vitamin C or iron supplements.⁵

Glucose meters and medicines

High intakes might cause falsely high or low readings on some blood glucose meters.⁶

Ask your pharmacist or doctor first

Prescribing labels for extended-release mixed amphetamine salts name ascorbic acid, which is vitamin C, as an example of an acidifying agent that lowers blood levels of amphetamine and can make it less effective. Tell your prescriber if you take vitamin C.^7,8

On the medicine's label

Ask your pharmacist or doctor first

Vitamin C taken at the same time as levothyroxine might increase how much of the medicine is absorbed.⁶

Reported in studies

Your doctor may want to check on you

In one study, vitamin C taken with other antioxidants such as vitamin E, selenium and beta-carotene blunted the rise in HDL cholesterol from simvastatin taken with niacin. It is not known whether this happens with other cholesterol medicines, and NIH says doctors should check cholesterol levels in people taking both.^1,2

Reported in studies

Vitamin C and other antioxidants might also interact with chemotherapy and radiation therapy. NIH advises people having cancer treatment to check with their oncologist first, especially before high doses.^1,2

Reported but not confirmed

Reports that high intakes lower vitamin B12 and copper, wear tooth enamel or cause allergic reactions have not been confirmed, and some came from errors in lab tests.¹

A few laboratory studies raised the idea that very high doses could add to oxidative damage rather than limit it. NIH notes that other studies have not shown more damage at high intakes.¹

Men and women

The upper limit is the same for men and women, 2,000 mg a day, and it does not change in pregnancy or while breastfeeding. Men need 90 mg a day and women 75 mg, so there is a wide margin between what the body needs and the limit.¹

Questions people ask

Is it OK to take vitamin C every day?

The recommended amounts are daily amounts, 75 mg for women and 90 mg for men. NIH defines the 2,000 mg upper limit as the most you can take each day without likely harm, counting food and supplements together.¹

What happens if you take too much vitamin C?

Loose stools, nausea and stomach cramps are the usual signs. High intakes also raise oxalate in urine, and in people with hemochromatosis, long-term high doses could add to iron overload.¹

Can vitamin C affect blood sugar readings?

It can affect some meters. NIH says high intakes might cause falsely high or low readings on some blood glucose meters, so tell your diabetes care team if you take vitamin C supplements.⁶

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Sources

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.